1. Field of the Invention
This invention relates generally to geophysical exploration and in particular to a vibratory seismic source useful in geophysical exploration. More particularly, the invention relates to a vibrator using a magnetostrictive driver.
2. Background Art
Seismic energy sources, including vibrators, are used in geophysical exploration on land and in water covered areas of the Earth. Acoustic energy generated by such sources travels downwardly into the Earth, is reflected from reflecting interfaces in the subsurface and is detected by seismic receivers, typically hydrophones or geophones, on or near the Earth's surface or water surface.
Most of the sources used today in land-based seismic survey operations are hydraulically actuated vibrators. Other examples of seismic energy sources include explosives and weight-drop impulse sources. The frequency content of such sources is controllable only to a limited degree, and different sources are selected for the generation of different frequency ranges of seismic energy for different surveying needs. Vibrator sources using piezoelectric or magnetostrictive materials as the actuating elements have been considered for use in land operations in order to provide better control over frequency content than is possible using hydraulic actuation. However, such sources have never been commercially deployed. Although such sources can generate signals over various frequency bands, commonly referred to as “frequency sweeps”, the limited power that such sources known to the art have been able to generate have limited their use in land operations.
It is well known that as acoustic waves travel through subsurface geological structures, higher frequency components of the acoustic waves are attenuated more rapidly than lower frequency components, and consequently, lower frequency sound waves can be transmitted over longer distances through geological structures than higher frequency acoustic waves. As such, there is a need in the technical fields of seismic surveying for powerful, controllable frequency, low frequency vibrator type acoustic sources.
One such seismic vibrator is described in U.S. Pat. No. 6,488,117, issued to Owen. The vibrator disclosed in the Owen patent includes a base plate and frame having a means for firmly and rigidly coupling the vibrator to the medium in which vertical dynamic forces produced by an integral electromechanical force driver generate seismic P waves in the ground. The disclosed vibrator also includes a means by which one, two, or more dynamic force driver units are attached to the frame in a manner such that their forces are efficiently transmitted as purely vertical forces to the ground coupling interface via the base plate; one, two, or more dynamic force driver units, operating either on the piezoelectric or magnetostriction force generating principle or on the electrodynamic force generating principle to mechanically excite directed forces on the base plate in the desired seismic source vibrator frequency range; one, two, or more inertial reaction masses suspended on the coupling frame by compliant springs and/or other isolation materials or components, these masses serving as inertial masses against which the force driver units react to apply dynamic forces to the frame and ground coupling base plate; and (5) frame components constructed integrally with the base plate to provide accurate and robust support of the reaction masses and compliant springs so as to avoid any tendencies for unwanted static deflections of the reaction masses or unwanted dynamic vibrations or tilting motions of the frame or base plate during vibrational operation. The described vibrator is “capable of generating vertically oriented forces in the ground to produce controlled seismic waveforms at frequencies typically up to 1,600 Hz and operating at moderate driving forces typically up to 1,000 lbf. This new seismic vibrator source is appropriately matched in size, cost, and mobility to applications in shallow geophysical and geotechnical field surveys”, however, the described vibrator is not intended for use in the same frequency range and force as hydraulically actuated vibrators.
Accordingly, there is a continuing need for improved vibrator type seismic energy sources that provide the frequency range and force of hydraulically.